Restoring force, in a physics context, is a variable force that gives rise to an equilibrium in a physical system. If the system is perturbed away from the equilibrium, the restoring force will tend to bring the system back toward equilibrium.
An example is the action of a spring: an idealized spring exerts a force that is proportional to the amount of deformation of the spring. Pulling the spring to a longer configuration causes it to exert a force that brings the spring back toward its equilibrium length. The amount of force can be determined by multiplying the spring constant of the spring by the amount of stretch.
Restoring forces do not necessarily cancel out a perturbation entirely. For example, taut piano strings have a strong restoring force that gives rise to an equilibrium when the string is straight. A struck piano string does not return directly to the straight equilibrium; rather, it oscillates, causing the piano to emit a particular musical note.